Development of improved high-strength polymeric fibers able to provide ballistic protection beyond conventional fibers has met with limited success. The developmental fiber poly(p-phenylene-2,6-benzobisoxazole) (PBO, Zylon®), which showed early promise in terms of strength, was later found to have poor aging and hydrolytic instability issues that severely compromised long-term effectiveness. Manufacturing rigid-rod polymers such as PBO and poly{diimidazo pyridinylene (dihydroxy) phenylene} (PPID, M5®) also presents significant processing obstacles. These include unavailability or oxidative instability of the starting materials, and removal of acidic residues and byproducts from the drawn fiber, which have been implicated in the poor hydrolytic stability of PBO (O'Neil. 2006). Recently investigated modifications of the PBO and PPID polymer backbones may increase intermolecular hydrogen bonding that could in turn potentially result in improved strengths (U.S. Pat. No. 3,767,756 to Blades; U.S. Pat. No. 3,869,429 to Blades).
Obtaining high strengths in organic fibers is dependent on the molecular weight, crystallinity, or molecular orientation of the constituent polymer, and its cohesive energy density. Highly ordered polymer fibers are obtained by optimization of the fiber drawing process. The magnitude of the cohesive energy density (CED) is driven by attractions between adjacent polymer molecules, which include dispersive, electrostatic, and hydrogen bonding forces. The most robust fibers will possess high relative crystallinity and an elevated CED. FIG. 1 illustrates the relationship between cohesive enemy, crystallinity, and tensile strength in selected polymers.
Polyurea coatings and composites have been gaining acceptance in ballistic and blast applications over the past several years. Additional evidence has emerged to indicate polyureas have superior performance compared to polyurethanes, particularly in response to blast and ballistic forces. This difference may be due to a higher number of hydrogen bonds in polyurea materials compared to polyurethanes. A recent report (Sheth et al., 2005) supports this supposition. Intermolecular cohesive forces are expected to be higher than those obtained in para-aramid synthetic fibers (e.g. Kevlar®),PBO and PPID based on preliminary modeling and calculations of cohesive energy density.